Impacts of silver nanoparticles on enzymatic activities,nitrifying bacteria,and nitrogen transformation in soil amended with ammonium and nitrate

Silver nanoparticles(AgNPs)are effective antimicrobial compounds that are used in a myriad of applications.Soil microorganisms play crucial roles in nitrogen cycling,but there is a lack of comprehensive understanding of the effects of Ag NPs on enzymatic activity in the nitrogen cycle,nitrifying bacteria,and nitrogen transformation in soil.Herein,enzyme activities were determined following the addition of different forms of nitrogen,ammonium nitrogen((NH_(4))_(2)SO_(4)),nitrate nitrogen(KNO_(3)),and amide nitrogen(urea,CO(NH_(2))_(2))at 200 mg N kg^(-1),into the soil amended with AgNPs at 0,10,50,and 100 mg kg^(-1).After 7 d of incubation with 10 mg kg^(-1) Ag NPs,the activities of urease,nitrite reductase(NiR),nitrate reductase(NaR),and hydroxylamine reductase(HyR)were reduced by 12.5%,25.0%,12.2%,and 24.2%,respectively.Of particular note,more than 53.5%,61.7%,and 34.7% of Na R,NiR,and HyR activities,respectively,were inhibited at 100 mg kg^(-1) Ag NPs.The abundance(most probable number)of ammonia-and nitrite-oxidizing bacteria(AOB and NOB,respectively)was measured using real-time quantitative polymerase chain reaction(qPCR)and the Cochran method.The abundance of AOB and NOB decreased when Ag NPs were present in the soil.The NH_(4)NO_(3) amendment increased copy numbers of bacterial and archaeal amo A nitrification functional genes,by 38.3% and 12.4%,respectively,but AgNPs at 50 mg kg^(-1) decreased these values by 70% and 56.4%,respectively.The results of ^(15)N enrichment(atom% excess)of NH_(4)^(+) and NO_(3)^(-) experiments illustrated the influence of AgNPs on soil nitrogen transformation.According to the ^(15)N atom% excess detected,the conversion of ^(15)N-labeled NH_(4)^(+) to NO_(3)^(-) was significantly inhibited by the different levels of Ag NPs in soil.The reduced gross nitrification rate further confirmed this finding.Overall,this study revealed considerable evidence that AgNPs inhibited nitrogen cycle enzyme activity,the number of nitrifying bacteria,the abundance of the amo A gene,and the gross nitrification rate.Silver nanoparticles inhibited nitrogen transformation,and the rate of nitrification was also negatively correlated with AgNP levels.